The present invention relates to abrasive particles, particularly cubic boron nitride (CBN) coated with a refractory metal oxide. The present invention also relates to grinding wheels made therefrom, especially vitreous bonded grinding wheels containing refractory metal oxide coated CBN particles as the abrasive material.
It is known in the art that the application of a metal coating to abrasive particles such as diamond and cubic boron nitride improves the retention of such particles in the various matrices of abrasive tools. For example, U.S. Pat. No. Re. 31,883 discloses that CBN particles coated with from 30 to 80 weight percent of nickel are useful in the manufacture of improved resin bonded grinding wheels.
U.S. Pat. No. 4,011,034 describes CBN particles having a rough, flaky granular covering consisting essentially of a coating ranging in composition from metal to a mixture of boride and nitride of the metal intermediate the surface coating and the CBN particles. This reference teaches that metal oxide coated CBN should be fired in a reducing atmosphere to convert the coating to nitrides and borides.
U.S. Pat. No. 4,399,167 pertains to a method of metal coating diamond and cubic boron nitride by providing a mass of metal powder of from 125 to 37 microns in contact with the abrasive particles so as to form a mixture consisting essentially of metal powder and diamond or CBN particles, heat treating such mixture in a non-oxidizing atmosphere at a temperature below the melting point of the metal to allow the metal to deposit on the particles, and thereafter breaking up the loosely sintered mass to recover the metal coated abrasive particles.
U.S. Pat. No. 3,528,788 discloses resin bonded grinding wheels having embedded therein diamond grit material coated with from 25 to 275 parts of metal per 100 parts of diamond, said metal being selected from Ni, Co, Ag, Cu, Mo, Ti, Al, Mn, Cd, Zn, Cr, V, Au, W, Fe, Zn and the Pt-group metals.
It is also known in the art that the retention of diamonds of the metal bond or saw type in metal matrices can be improved by providing the particles with a double layer coating, the inner layer adjacent the particle being a carbide former such as titanium end and the outer layer being a metal which alloys with the carbide former, for example, as disclosed in U.S. Pat. Nos. 3,826,630 and 3,929,432.
There still remains, however, the need for even more improved coated abrasive particles and grinding wheels made therefrom. Moreover, despite the numerous improvements in resin bonded grinding wheels and metal bonded grinding wheels, there have not been similar advances in respect of vitreous bonded grinding wheels, and particularly vitreous bonded grinding wheels containing cubic boron nitride as the abrasive material.
Such CBN containing vitreous bonded grinding wheels are very effective for high removal rates of steel in precision and form grinding. Unfortunately, the conventional glass frits used in preparing the bond matrix attack the CBN and cause bloating and slumping. It has now been discovered that by coating the CBN abrasive particles with a refractory metal oxide coating, the shortcomings of prior art vitreous bonded grinding wheels can be overcome.